The present subject matter relates to stop-start fuel saving systems for vocational vehicles powered by a conventional internal combustion engine and powertrain.
In recent years, much effort has been expended by the automotive industry to develop automobiles with reduced fuel consumption and lower exhaust emissions. A variety of technologies have been explored, including regenerative braking, hybrid electric propulsion systems, and plug-in electric propulsion systems.
One technology that has recently been adopted by a number of automobile manufacturers is stop-start systems for vehicles having a conventional internal combustion engine and powertrain. Stop-start systems save fuel by shutting off the internal combustion engine when the vehicle has been braked to a stop, such as at a traffic light, and restarts the engine when the driver disengages the brake and steps on the accelerator. Stop-start systems reduce the amount of time that the engine is idling while the vehicle is stopped. They therefore reduce fuel consumption, reduce exhaust emissions, reduce engine wear, and reduce noise. Stop-start systems are relatively inexpensive compared to hybrid power systems or plug-in electric power systems.
Nevertheless, stop-start systems present a number of design challenges for vehicles having a conventional internal combustion engine and powertrain. For one thing, neither the battery nor the starter motor of a conventional vehicle would be able to handle the repeated stop-start cycles as would be experienced when driving in busy city traffic. In addition, redesign or modification of automatic transmissions is generally required to ensure adequate responsiveness when a stop-start system restarts the internal combustion engine.
Several different types of stop-start systems have been developed by manufacturers of consumer passenger vehicles. A summary of some of these is provided in the report by FEV GmbH entitled “In-Market Application of Stop-start Systems in European Market” (Markus Kremer), Dec. 6, 2011.
Notwithstanding the increasing adoption of stop-start systems for automobiles, there has so far been little deployment among vocational vehicles such as refuse collection trucks, bucket trucks, terminal tractors, dump trucks, cement mixer trucks, urban buses, and parcel delivery trucks. This may be considered surprising because, unlike most automobiles, many vocational vehicles are subject to frequent stops and re-starts as part of their regular duty cycle, even when there is no heavy traffic or dense distribution of traffic lights or stop signs. It is considered that there are at least three factors that have hindered the widespread adoption of stop-start systems for vocational vehicles.
For one thing, as mentioned above, developing a stop-start system involves a number of design challenges, requiring extensive development and testing; it may well be that the relatively small markets for specialized vocational vehicles has hindered the commitment of resources to develop stop-start systems for such vehicles.
Another factor pertains to the nature of the vocational vehicle industry. Automobile manufacturers typically develop, engineer and assemble the entire vehicle. By contrast, specialized vocational vehicles usually have several manufacturers: one or more manufacturers design and make the rolling chassis, which includes the frame and powertrain, while a different manufacturer designs and builds the body, which usually includes various specialized operating features and systems. Because of the lack of integration of the several manufacturing steps, it may be difficult to develop new technologies that involve design criteria affecting both the body and the rolling chassis.
A third aspect relates to the fact that the bodies of many vocational vehicles incorporate pieces of auxiliary equipment to execute a working function, such as the bin lifting arm of a garbage truck, the elevating boom of a bucket truck, the rotation of the drum on a cement mixer truck, and/or the bed lifter of a dump truck. Such equipment requires a mechanical drive, often to power a hydraulic pump, which is generally provided by the engine through a power take-off interface on the transmission or on a mechanical interface on the crankshaft of the engine. When the engine has been turned-off, the auxiliary equipment cannot be operated.
In some cases, it has been proposed to provide a battery pack on the vehicle to provide a secondary source of energy to drive auxiliary working equipment such as an electric motor to operate a hydraulic pump. The battery pack must be sized in a manner to get a sufficient amount of energy to supply the equipment for an entire day of operation of the vehicle, during the periods when the stop-start system has turned-off the engine. Typically, such a configuration requires a large battery that is recharged by being plugged-in to electric mains overnight; such a vehicle is commonly referred to as plug-in hybrid electric vehicle (PHEV).
The energy consumption of auxiliary systems on vocational vehicles can be very significant during the course of a typical work day. As an example, the compactor on a refuse truck requires at least 300 kJ per cycle of compaction, representing over 30 kWh per day. Moreover, to achieve any given amount of usable energy, a battery should have about twice the capacity to provide adequate battery life.
Assuming one full charge-discharge cycle per day, a vocational vehicle would require a battery with a minimum lifespan of at least 2500 cycles to meet an expected vehicle life of 10 years. The lifespan of lead acid batteries is well below this target. Although lithium-ion batteries can reach a lifespan of 2500 cycles, meeting the required energy capacity with lithium-ion batteries is currently expensive, and the batteries themselves add significant weight, which of course increases fuel consumption when the vehicle is being driven.
Vehicles with hybrid power systems generally incorporate a large lithium-ion battery pack, making it easier to incorporate a stop-start function with the capacity to power auxiliary equipment using the energy stored in the battery pack.
The present inventors' U.S. Pat. Nos. 8,840,524 and 9,132,824 describe several embodiments of a stop-start system that has recently gathered interest among operators of heavy duty vocational vehicles such as refuse trucks. The entire contents of U.S. Pat. Nos. 8,840,524 and 9,132,824 are hereby incorporated by reference.
One of the embodiments described therein relies on an electric energy storage device which can be used to: power an electric motor that drives a hydraulic pump to maintain hydraulic pressure in the automatic transmission when the engine has been turned off by the system; and power a restarting motor that restarts the engine while the transmission is in gear; and also power a motor to drive a pump to operate an auxiliary hydraulic system on the vehicle, such as the bin lifter of a refuse collection vehicle, while the engine has been turned off by the system.
While many stop-start systems use lithium-ion batteries, the present inventors have recognized that in certain applications, it is advantageous to rely on an electric energy storage device characterized by relatively low storage capacity, and with relatively rapid discharge-recharge times but high cycle life, compared to currently available lithium-ion batteries. Currently, Electric Double Layer Capacitors (aka EDLCs, or ultracapacitors, or super-capacitors), are available that meet these characteristics.
Suitable ultracapacitors selected for vocational vehicle stop-start systems may have an energy storage capacity between about 100 to 500 Wh, which should be sufficient to operate most auxiliary equipment during times when the internal combustion engine has been shut-off, and also to restart the engine. Such ultracapacitors should also have a high power capacity to allow powering of equipment rated at something in the order of 10 kW, and also allowing them to be recharged quickly, advantageously in less than one minute. Such ultracapacitors should also have a lifespan in excess of about 1 million cycles to remain operational for a vehicle life expectancy of 10 years. In addition, they should also be compatible with the environmental variables of the specific vocational vehicle, including operating temperature range, corrosion resistance and vibration resistance.
The present inventors have recognized that ultracapacitors with these characteristics can be very effective in stop-start systems deployed on refuse vehicles, as one example, given their high frequency of brief stops, and short travel times between stops. Unlike a lithium-ion battery or a nickel metal hydride battery, ultracapacitors can easily go through hundreds of discharge and recharge cycles in a day, while still being expected to maintain full functionality throughout the life of the vehicle.
Given that ultracapacitors can sustain charge-discharge cycles in excess of one million cycles, the energy storage capacity can be reduced to as low as the energy required to perform one operation cycle of the equipment of the vehicle plus the energy required to re-start the engine. The size of the energy storage can thus be reduced by a factor of up to at least one hundred compared to systems powered by lithium-ion batteries. Consequently, ultracapacitors allow the stop-start system to be smaller, lighter and less expensive than a system that relies on lithium-ion batteries.
While a stop-start system relying on ultracapacitors (or other relatively low energy capacity electric energy storage devices having relatively low storage capacity but rapid recharge times and long cycle life characteristics) has a number of advantages in certain vocational vehicle applications, particularly for vehicles having very frequent and short stops where auxiliary equipment is used, such as refuse vehicles, it has now been recognized by the inventors that such a stop-start system can provide additional functionalities.